1. Field of the Invention
This invention relates generally to the field of wireless communications. In particular, the invention relates to a method and apparatus for interfacing the Global Positioning System (“GPS”) devices to different communication devices independent of any aiding specific protocols emanating from the communication devices.
2. Related Art
The worldwide utilization of wireless devices (also known as “mobile devices”) such as two-way radios, portable televisions, Personal Digital Assistants (“PDAs”), cellular telephones (also known as “wireless telephones,” “wireless phones,” “mobile telephones,” “mobile phones,” and/or “mobile stations”), satellite radio receivers and Satellite Positioning Systems (“SATPS”) such as the Global Positioning System (“GPS”), also known as NAVSTAR, is growing at a rapid pace. As the number of people employing wireless devices increases, the number of features offered by wireless service providers also increases, as does the integration of these wireless devices with other products.
Since the creation of the NAVSTAR GPS system by the U.S. Department of Defense (“DoD”) in the early 1970s, numerous civilian applications have arisen that utilize new technologies associated with GPS. These new technologies include, as examples, personal GPS receivers that allow users to determine their positions on the surface of the Earth, and numerous communication networks such as the Code Division Multiple Access (CDMA) and Time Division Multiple Access (TDMA) cellular networks that utilize GPS clock references to operate. As a result of these new technologies, there is a growing demand for mobile communication devices that can transmit, among other things, their locations in emergency situations, incorporate positional information with communication devices, locate and track tourists, children and the elderly, and provide security for valuable assets.
In general, GPS systems are typically satellite (also known as “space vehicle” or “SV”) based navigation systems. Examples of GPS include but are not limited to the United States (“U.S.”) Navy Navigation Satellite System (“NNSS”) (also know as TRANSIT), LORAN, Shoran, Decca, TACAN, the Joint Program Office (“WO”) Global Positioning System known as NAVSTAR, which was developed by the Department of Defense (DoD), the Russian counterpart known as Global Navigation Satellite System (“GLONASS”) and any future Western European GPS such as the proposed “Galileo” program. The NAVSTAR GPS (henceforth referred to simply as “GPS”) was originally developed as a military system to fulfill the needs of the U.S. military; however, the U.S. Congress later directed the DoD to also promote GPS's civilian uses. As a result, GPS is now a dual-use system that may be accessed by both U.S. government agencies (such as the military) and civilians. The GPS system is described in Global Positioning System: Theory and Practice, fifth, revised ed., by Hofmann-Wellenhof, Lichtenegger and Collins; Springer-Verlag, Wien, N.Y., 2001, which is fully incorporated herein by reference.
Typically, the utilization of GPS includes identifying precise locations on the Earth and synchronizing telecommunication networks such as military communication networks and the cellular telephone networks such as CDMA and TDMA type systems. Additionally, with the advent of the United States Congress' mandate, through the Federal Communications Commission (“FCC”), for a cellular telephone network that is capable of providing a cellular telephone user's location within 50 feet in emergency situations (generally known as “Enhanced 911” service or “E911”), GPS is being employed for both location determination and synchronization in many cellular applications.
In general, the array of GPS satellites (generally known as a “GPS constellation”) transmit highly accurate, time coded information that permits a GPS receiver to calculate its location in terms of latitude and longitude on Earth as well as the altitude above sea level. GPS is designed to provide a base navigation system with accuracy within approximately 100 meters for non-military users and even greater precision for the military and other authorized users (with Selective Availability “SA” set to ON).
In general, GPS comprises three major system segments: space, control, and user. The space segment of GPS is a constellation of satellites orbiting above the earth that contain transmitters, which send highly accurate timing information to GPS receivers on earth. At present, the implemented GPS constellation includes 21 main operational satellites plus three active spare satellites. These satellites are arranged in six orbits, each orbit containing three or four satellites. The orbital planes form a 55° angle with the equator. The satellites orbit at a height of approximately 10,898 nautical miles (20,200 kilometers) above the Earth with orbital periods for each satellite of approximately 12 hours.
Generally, each of the orbiting satellites contains four highly accurate atomic clocks (two rubidium and two cesium). These atomic clocks provide precision timing pulses used to generate a unique binary code (also known as a pseudorandom “PRN-code” or pseudo noise “PN-code”) that is transmitted to Earth. The PRN-code identifies the specific satellite in the GPS constellation. The satellite also transmits a set of digitally coded information that includes two types of orbital parameters for determining the locations-in-space for the satellites known as almanac data and ephemeris data.
The ephemeris data (also known as “ephemerides”) defines the precise orbit of the satellite. The ephemeris data indicates where the satellite is at any given time, and its location may be specified in terms of the satellite ground track in precise latitude and longitude measurements. The information in the ephemeris data is coded and transmitted from the satellite providing an accurate indication of the position of the satellite above the Earth at any given time. Typically, current ephemeris data is sufficient for determining locations in space to a few meters or a few tenths of meters at current levels of SA. A ground control station updates the Ephemeris data each hour to ensure accuracy. However, after about two hours the accuracy of the ephemeris data begins to degrade.
The almanac data is a subset of the ephemeris data. The almanac data includes less accurate information regarding the location of all the satellites in the constellation. The almanac data includes relatively few parameters and is generally sufficient for determining locations-in-space to a few kilometers. Each GPS satellite broadcasts the almanac data for all the GPS satellites in the GPS constellation on a twelve and one-half (“12.5”) minute cycle. Therefore, by tracking only one satellite, the almanac data of all the other satellites in orbit are obtained. The almanac data is updated every few days and is useful up to approximately several months. Because of its relatively long lifetime, GPS receivers that have been off for more than a few hours typically utilize the almanac data to determine which GPS satellites are in-view. However, both the almanac and ephemeris data are valid only for a limited amount of time. As such, the location of the satellites based on this information is less and less accurate as the almanac and ephemeris data ages unless the data is updated at appropriate intervals in time.
The ephemeris data includes three sets of data available to determine position and velocity vectors of the satellites in a terrestrial reference frame at any instant. These three sets of data include almanac data, broadcast ephemerides, and precise ephemerides. The data differs in accuracy and is either available in real-time or after the fact. Typically, the purpose of the almanac data is to provide the user with less precise data to facilitate receiver satellite search or for planning tasks such as the computation of visibility charts. The almanac data are updated at least every six days and are broadcast as part of the satellite message. The almanac message essentially contains parameters for the orbit and satellite clock correction terms for all satellites. The GPS almanac data is described in “GPS Interface Control Document ICD-GPS-200” for the “NAVSTAR GPS Space Segment and Navigation User Interfaces” published by NavTech Seminars & NavTech Book and Software Store, Arlington, Va., reprinted February, 1995, which is herein incorporated by reference.
In a typical operation example, when a GPS receiver is first turned on (generally known as a “cold start”) or woken up from a long stand-by condition of more than a few hours, the GPS receiver will scan the GPS spectrum to acquire a GPS signal transmitted from an available GPS satellite. Once the GPS signal is acquired the GPS receiver will then download the GPS almanac data for the GPS constellation, the ephemeris data and clock correction information from the acquired GPS satellite. Once the almanac data is downloaded, the GPS satellite will then scan the GPS spectrum for the available (i.e., the “in-view”) GPS satellites as indicated by the almanac data. Ideally, given sufficient time and assuming the environmental conditions surrounding the GPS receiver allow the GPS receiver to acquire two to three additional in-view GPS satellites, the GPS receiver receives both distance and timing information from the three to four satellites and calculates its position on the Earth.
Unfortunately, for many applications both time and environmental conditions may limit a GPS receiver's ability to download the GPS almanac data, especially in indoor or limited sky-view conditions. The problems associated with time are usually described by the Time-to-First-Fix (“TTFF”) values. If the TTFF values are high, the GPS receiver will have limited applications because it will take too long to determine its initial location.
As an example, in a wireless or mobile (such as a cellular) telephone application, a mobile telephone or personal digital assistant (“PDA”) with an integrated GPS receiver may have to wait approximately 12.5 minutes (assuming perfect environmental conditions with all necessary in-view satellites being visible) for the GPS receiver to download the GPS almanac before making a call. This would be unacceptable for most applications.
In cellular telephone applications, this limitation is even more unacceptable in view of the E911 mandate that requires that a cellular telephone send its position information to emergency personal in an E911 emergency call. If users find themselves in an emergency situation with a GPS enabled cellular telephone that is turned off or in a long stand-by condition, those users would have to generally first wait for approximately 12.5 minutes of time with continuous uninterrupted satellite visibility (because the GPS receiver typically needs a strong signal to acquire the almanac and/or ephemeris data reliably) before being able to make an emergency call that would transmit the user's location to the emergency personal. In typical metropolitan or naturally obstructed environments, this wait may be longer than 12.5 minutes because the environmental conditions may make acquiring the first satellite more difficult. It is appreciated that this would be unacceptable, especially in a life-threatening situation.
Past approaches to reduce the amount of time required to download the almanac data have included storing some sort of almanac (such as factory installed almanac data) in a memory unit (such as a read-only memory “ROM”) in the GPS receiver. Typically, this pre-stored almanac data is utilized to reduce the TTFF in a cold-start condition. In this approach, the cold-start condition usually still has a relatively long TTFF time due to the uncertainties associated with the satellite positions and the age of the pre-stored almanac. Once the first fix is acquired, this GPS receiver may then download the updated almanac data from the acquired satellite and update the ROM (or a read-access memory “RAM”) for future use. However, this approach still requires that the GPS receiver receive the updated almanac data (i.e., receiving a “fresh” copy of the almanac data) from the satellites for future acquisitions. Receiving the updated almanac data will still require significant amounts of time that will affect the performance of the GPS receiver.
In response to these problems, aiding approaches have been developed for mobile telephones that assist the GPS receiver by providing aiding data from a communication module (also known as a “call processor” or “CP”) for such purposes as acquisition, location calculation and/or sensitivity improvement). Unfortunately, these aiding approaches in wireless networks are typically cellular network (i.e., cellular platforms such as TDMA, GSM, CDMA, etc.) and vendor specific, and are provided by Geolocation Server Stations located at the cellular network. As a result, the GPS receiver in the mobile telephone (also known as a “mobile station” or “MS”) must typically be compatible with the Geolocation Server Station of the cellular network.
However, there are numerous cellular networks in operation throughout the United States and abroad that either incorporate, or will incorporate, Geolocation Server Stations that utilize Geolocation Server Station protocols that are not compatible with each other. Therefore, there is a need for a system capable of allowing a GPS receiver to operate with the numerous Geolocation Server Stations that is Geolocation Server Station protocol independent.